Recently, demand for a technology for storing a large amount of information at a high speed has increased along with an increase in the amount of information. The storage density of magnetic recording, which is currently the most widely used as means for recording information, is approaching the theoretical limit. Even when vertical magnetic recording is used, it is believed that 1 Tbit/inch2 is the upper storage density limit. On the other hand, a ferroelectric exhibits spontaneous polarization, the direction of which can be reversed by applying an electric field from the outside to the ferroelectric. Accordingly, it is possible to record information by associating corresponding digital data with the direction of polarization of the ferroelectric. In addition, a domain wall of the ferroelectric has a thickness of about 1 or 2 lattice units and is significantly thinner than that of the ferromagnetic as is well known. Since the domain size of the ferroelectric is also much smaller than that of the ferromagnetic, it is believed that it will be possible to obtain an ultrahigh-density storage device if it is possible to control such microscopic domains of the ferroelectric. However, it is difficult to measure inner polarizations of the ferroelectric, i.e., to read information recorded in the ferroelectric since the inner polarizations of the ferroelectric are shielded by surface charges on the ferroelectric such as electrons or ions attached to the surface of the ferroelectric.
A Scanning Nonlinear Dielectric Microscope (SNDM) is known as a device for purely electrically detecting the distribution of polarization of a ferroelectric. FIG. 1 is a block diagram of a conventional device for detecting the direction of polarization of a ferroelectric, to which the SNDM is applied. This device determines the direction of polarization of a ferroelectric material 1 by measuring the nonlinear dielectric constant of the ferroelectric material 1, i.e., capacitance Cp thereof directly below a probe 3. In this device, to detect the direction of polarization of the ferroelectric material 1, an alternating electric field Ep is applied between a stage 2 and both a link probe 4 and a probe 3. Thus, the oscillation frequency of an oscillator 5 changes according to the alternating electric field. Since the rate of the change of the oscillation frequency including the sign is determined by the nonlinear dielectric constant (i.e., the capacitance Cp) directly below the probe, the probe 3 detects the distribution of polarization of the ferroelectric material 1 by performing 2D scanning on the ferroelectric material 1. After the change of the frequency of the oscillator 5 is demodulated by an FM demodulator 6, the frequency change is detected through synchronous detection using the frequency of the applied electric field at a PSK demodulator 7.
Patent Reference 1:
    Japanese Patent Kokai No. 2004-127489